A current trend in the industry is the continuing migration of sensitive electronic systems, which currently reside within environmentally controlled buildings, into remote enclosures located in thermally uncontrolled environments. The operating conditions for these remote electronic systems are formidable and significantly more challenging than those faced by central office equipment. Remote equipment must function under a wide range of ambient temperature conditions and endure the added stress of diurnal ambient temperatures and solar cycling. In addition, dust, humidity, pollutants, vibrations from traffic and power surges (including lightning strikes) in these thermally uncontrolled environments conspire to create an environment that will accelerate failure mechanisms in these remote electronic systems.
Further compounding the issue of increased environmental stresses are the demanding reliability requirements faced by these remote systems. By distributing electronic subsystems at remote locations throughout the network, the mean time to repair any one of them will increase in comparison to an equivalent repair to centrally located equipment. This is due to the time required to travel to the site of the failure and the increased time to repair the failure under potentially uncontrolled conditions, (e.g., rain, snow). As a result, remote electronic systems are required to achieve reliability requirements far in excess of those demanded of central office equipment in order to maintain a comparable level of network availability.
The high cost of providing power to remote electronic systems will also tend to discourage the use of active environmental conditioning systems, (e.g., air conditioners, heaters, heat exchangers). These conditioning systems tend to consume large amounts of power and, in many cases, will limit overall system reliability as they are often found to be less reliable than the electronic systems that they were designed to protect.
Another challenge to the reliability of remote electronic systems is the continuing pace of development in the electronics industry that has led to rapid improvements in integrated circuit performance in recent years, with resultant increases in total chip power dissipation and power densities for advanced devices. Industry estimates for the year 2004 forecast that total chip power dissipation for commercial electronic devices of between 40 to 120 Watts with power densities of 50 to 100 W/cm.sup.2 will be common.
As a result, thermal management and its influence on system reliability represents one of the most challenging issues in the development of modern cost-effective remote electronic systems.
It is well known that liquid immersion cooling is an effective method of controlling the temperature of electronic systems. Studies of liquid immersion cooling dating back to the 1950's and 60's demonstrated effective thermal control of large electronic devices (e.g., travelling wave tubes and high voltage power supplies). Later, liquid immersion cooling was introduced to various electronic components and immersion cooled chips. Among liquid immersion cooling modules, passive liquid immersion modules tend to be the most reliable, simple in design, and requiring minimum maintenance efforts. In a passive liquid immersion cooling apparatus, electronic components are immersed in a container of a dielectric liquid having a low boiling point, which is usually sealed from ambient atmosphere. The mode of cooling and consequently the heat transfer is dependent on the heat flux at the surface interface between heat generating components and the cooling liquid. For a small heat flux which creates a temperature below the boiling point of the liquid, natural convection takes place. As the heat flux increases the temperature beyond the boiling point of the liquid, nucleate boiling takes place. The nucleate boiling causes vaporization of the liquid immediately adjacent to the hot component. As the vapor bubbles form and grow on the heated surface, they cause intense micro-convection currents. Thus, nucleate boiling gives rise to an increase in convection within the liquid and, as a result, improves the heat transfer between the hot surface and the liquid. As the heat flux increases, film boiling occurs as the number of bubbles increases to the point where they begin to coalesce and the limiting heat flux commonly known as "critical heat flux" is reached. This point is considered as the practical limit for cooling electronics.
The modes of boiling or heat transfer discussed above have proven to be very efficient. For example, U.S. Pat. No. 3,489,207 to Miller, 1970 provides an indoor vapor-cooled electronics enclosures within a rectangular sealed container partially filled with a vaporizable heat exchange fluid to a level, which covers the electronic components that give off most heat during operation, and wherein electronic components giving off less heat are suspended in the container above the liquid level. The liquid contacts all major surfaces of the submerged electronic components and undergoes a boiling condensation cycle to effect heat removal.
U.S. Pat. No. 3,741,292 to Aakalu, 1973 provides a liquid encapsulated module of a parallelepiped shape which contains a plurality of heat generating components mounted on a substrate to which a container is attached in sealed relationship such that the components are exposed to the inside of the container. A low boiling point dielectric liquid partially fills the container and completely covers the components. A vapor space is located above the liquid level within the container. Internal fins extend inward within the container serving as a condenser for the vapours. External fins extend outwardly of the container serving as an air cooled sink for the internal fin condenser.
An article "Chips Immersion Cooled Inside Device Package" by Howard W. Markstein, published in Electronic Packaging & Production, January 1993, p. 33, describes high heat producing chips which are individually cooled in their device package by liquid immersion. The sealed chip cavity is partially filled with a dielectric fluid and a vapor/condensate cycle transfers heat to either a metal bellows cover or a finned cover of ceramic or metal. The immersed chip acts as the evaporator and the cover as the condenser.
However, the references cited above and other known applications of liquid immersion cooling have been designed to efficiently remove heat from electronic systems operating in fixed ambient temperature environments. For thermally uncontrolled operation, such as an outdoor operation, it is necessary to have a cooling apparatus which could be able to provide exploitation over an extended range of temperatures. Firstly, similar to indoor applications, it is necessary for the cooling apparatus, operating outdoors, to remove heat as efficiently as possible at high external ambient temperatures and/or high internal heat dissipations to maintain the temperature of electronic components at a low temperature rise above the ambient temperature. Secondly, it is necessary for the same cooling apparatus, to remove heat at low external ambient temperatures in a less efficient manner to maintain the temperature of the electronic components at a higher temperature rise above the ambient temperature. Additionally, cooling apparatus for outdoor environments must also provide high reliability and efficiency. It will be appreciated, in order to ensure that the cooling apparatus is useful for outdoor applications, it has to be conveniently designed to provide its operation over a wide range of temperatures, to provide reliable sealing, vapor pressure containment, protection from thermal and solar radiation, and assembly and replacement of different types of cooling apparatus.